This invention relates to an electrophotographic apparatus and method for preventing a charging potential at a photoreceptor from being decreased.
In an electrophotographic apparatus, such as an ordinary photo copying machine and laser beam printer, the surface of a photoreceptor is charged through a corona discharge, and either one of an electron and hole in a pair created in the photoreceptor upon an image exposure is neutralized with a charge on the surface of the photoreceptor to form an electrostatic latent image on the surface of the photoreceptor. The latent image is developed into a visual image by depositing a toner on the latent image area of the photoreceptor in which case the toner is charged with a polarity opposite to that of the charge on the surface of the photoreceptor. Then the developed image is transferred to a copying sheet to obtain a copy image on the copying sheet. After the toner on the surface of the photoreceptor has been removed therefrom, the latent image on the surface of the photoreceptor is erased in readiness for the next image forming cycle. This step is called as a discharging step. This discharging step is performed by generally illuminating the whole surface of the photoreceptor with light.
As a material for the photoreceptor, use may be made of a chalcognide series, such as selenium, and an organic semiconductor, such as polyvinyl carbazole (PVK), but these materials have a low sensitivity to a visible light ray. These materials have a lower strength, a lower temperature stability, etc., as well as a shorter service life when they are applied to the electrophotographic photoreceptor.
Recently attention has been paid to amorphous silicon (hereinafter referred to as a-Si) which is employed as a material for the photoreceptor. The a-Si represents a high and panchromatic sensitivity, an excellent temperature stability, a higher Vickers' hardness of about 1000 and a longer service life. The electrophotographic apparatus employing a photoreceptor including a-Si can reduce the time taken for one step of electrophotography, in comparison with that taken on a conventional apparatus. This advantage is obtained particularly from the broader photosensitive wavelength range and higher spectral sensitivity. That is, the photoreceptor of the a-Si is effective for a high-speed copying operation.
In the conventional copying process, however, if use is made of the a-Si type photoreceptor, a charging potential level on the photoreceptor is significantly lowered at the discharging step.
This is a phenomenon inherent in the a-Si. When, in the band gap, electrons are induced at a localized level in the neighborhood of a conduction band, they do not promptly neutralize charges at the surface of the photoreceptor. When the electrons are further excited by heat and rise above the localized level, they neutralizes the charges.
Hence, the electrons excited at the localized level through the absorption of the light component of a long waveform of the discharge light do not contribute to the discharging and, after having been excited thermally, neutralize ions at the next charging step. If, in particular, the charging and discharging means are located in proximity to each other, this phenomenon promptly emerges due to the ready light leakage onto the photoreceptor surface.
As a result even if, in the next charging step, the same voltage as a previous one is applied, only 50% to 80% of that charging voltage generated in the dark is obtained. If, therefore, the charging voltage is thus lowered, then the image contrast becomes smaller, resulting in a poorer image quality.